Visual space perception in the intermediate distance range (2-25 meters) serves a multi-faceted function. It allows the observer to perceive the 3D shapes, sizes and locations of objects, orient in space and guide our actions. Yet, despite its importance in enabling the observer to interact in the natural environment, knowledge of the mechanisms underlying space perception in the intermediate distance is incomplete. Pertaining to the ability to locate an object on the ground, previous work found that the visual system achieves this by constructing a representation of the ground surface and then uses it as a reference frame to localize the object. To localize a suspended object requires determining the relative position between the suspended object and the ground reference frame. Recent empirical studies have provided critical evidence for the notion of a ground-based space perception, and led to the discovery of several fundamental mechanisms for representing the ground surface. This progress has motivated the formulation of a more comprehensive theoretical framework for understanding how the visual system forms the ground-based perceptual space. The goal of the current proposal is to seek quantifiable empirical proofs to substantiate and advance the framework. The following three broadly defined issues will be investigated: A. The Perceptual Mechanisms for Representing the Ground Surface and Ceiling Surface B. The Selection Hypothesis and Attention Operation in Space Perception C. Locating an Object that Does Not Have a Direct Surface Contact with the Ground The knowledge gained from the proposed project will advance the science of human space perception in the intermediate distance range. It will also improve understanding of the space perception and action problems encountered by patients with visual deficits (e.g., those with strabismus and amblyopia), as well as problems encountered by ordinary people in non-optimal visual environments. Furthermore, to complement the research in the real environment, the proposed project will investigate space perception in the virtual environment by capitalizing on the virtual reality technology. This approach is not only useful for controlling the experimental stimulation, but will also yield insights into how one can improve the immersive quality of virtual scenes.